The invention concerns an electronic balance with a measured value receiver which emits an analogous voltage or current signal with a high-resolution digitizer on the output side, with a digital signal processing unit, e.g. in the form of a microprocessor, and with means for correcting temperature errors of the measured value receiver.
A balance of this type is known, e.g. from DE-OS No. 30 33 272. The correction of temperature errors of the zero point is achieved in it by a current controlled in dependency on the temperature which produces in cooperation with a magnetic field a temperature-dependent additional force on the measured value receiver. However, this direct intervention in the measured value receiver is complicated, especially if not only the temperature error of the zero point but also any temperature error of the sensitivity or of the linearity should be corrected. It is therefore obvious to shift the correcting of the temperature errors to the digital signal processing unit, since all temperature-dependent corrections can be calculated there with one temperature measured value. DE-OS No. 31 06 534 therefore suggested providing separate temperature, moisture and pressure sensors, the analogous output signals of which are fed in succession to a digitizer. The digital signal processing unit can then calculate the necessary corrections from the digitized values. The disadvantage of this, however, is the additional expense for the separate digitizer on the output side of the sensors.
The obvious solution of feeding the output signal of the actual measured value receiver of the balance and the output signal of the temperature sensor(s) in succession over selector switches to the high-resolution digitizer can not be accomplished, because a switch in the measuring path from the measured value receiver to the digitizer would very adversely effect the preciseness of the balance.
The invention has the task of digitizing the analogous output signal of one or more temperature sensors (or of one or more similar sensors) without having to use an additional digitizer and without adversely affecting the preciseness of the digitizing of the output signal of the actual measured value receiver of the balance.
The invention solves this task by providing a switch which feeds the output signal of at least one temperature sensor to the input of the digitizer in addition to the signal of the measured value receiver at certain times controlled by the digital signal processing unit.
Therefore, the sensitive measuring path from the measured value receiver of the balance to the high-resolution digitizer is not interrupted and the output signal of the temperature sensor (s) is fed in addition to the input of the digitizer at certain times. The digital signal processing unit on the output side can then compute the output signal of the temperature sensor (s).
It is advantageous if the temperature sensors are dimensioned and connected in such a manner that their maximum output signal at the input of the digitizer is at least 10 times smaller than the maximum signal of the actual measured value receiver of the balance. This means that the measuring range of the digitizer is hardly limited at all for the actual measured value receiver of the balance. Nevertheless, the measuring preciseness of the temperature measuring is sufficient on account of the high resolution of the digitizer. In a balance with e.g. 100,000 increments resolution the temperature sensors are normally at 200 increments at the most. Thus, a preciseness of 1 part per thousand is amply sufficient for measuring the temperature, which is achieved in a digitizer with 100,000 increments resolution as early as at an input signal of 1% of the maximum input signal.
It is advantageous to construct the input of the digitizer as a current dip. This lets the summation of the two signals from the measured value receiver and from the temperature sensor (s) be performed without feedback as current summation.
It is advantageous if the digital signal processing unit contains a standstill monitoring circuit and actuates the switch which feeds the output signal of at least one temperature signal to the input of the digitizer in addition to the signal of the measured value receiver only when the standstill monitoring circuit announces a standstill. For the measuring of the output signal of the temperature sensors is not falsified by changes of the signal of the actual measured value receiver only when the balance is at a standstill.
It is advantageous if the output signal of the temperature sensor (s) is calculated by the signal processing unit according to the formula EQU U.sub.T =U.sub.1 -1/2(U.sub.0 +U.sub.2).
In this formula U.sub.0 represents the output value of the digitizer before the switch is actuated, U.sub.1 is the corresponding signal during the actuation phase of the switch and U.sub.2 is the corresponding signal after the end of the actuation phase of the switch. Thus, the difference of the signal during the actuation of the switch and of the average value of the two signals before and after is formed. It is advantageous if all values U.sub.0, U.sub.1 and U.sub.2 are not measured until the standstill monitoring circuit is actuated. It is advantageous if the digital signal processing unit additionally checks whether the initial value U.sub.0 and the final value U.sub.2 coincide within a certain error tolerance and evaluates the calculated value for U.sub.T only in this instance. If this means that a value for U.sub.T can be calculated and evaluated only relatively infrequently in comparison to the measuring cycle of the digitizer, this does not pose a problem, since the temperature in the balance changes only very slowly due to its heat inertia. An analogous situation applies to moisture sensors or air pressure sensors, which can of course be present in addition to the temperature sensors or instead of the temperature sensors, and the output signal of which can be digitized and calculated in the manner described.